Pathogenicity and Morphology of Paragonimus Westermani

Paragonimus Westermani is extensively distributed in the Far East, and its also found in West Africa and South America.

Habitat of Paragonimus Westermani 

(a) The Adults of paragonimus westermani live in the bronchi, lungs and occasionally in the liver, spleen and other organs of man, dog, cat, tiger, wolf, beaver, mongoose and pig.

(b) The Eggs are present in sputum and more rarely in feces. It is infective only to the snails.

(c)  The first larval host, in which sporocysts, rediae, and young cercariae develop is the fresh-water mollusc Semisulcospira libertine and other species of the same genus.

(d)  The second larval host in which metacercariae occur is the fresh-water crabs and crayfish.

Morphological Characters of Paragonimus Westermani 

Fleshy worms, reddish-brown in colour when fresh, in shape resembling one half of a pea and covered with short spines; they measure up to 1.2cm in length, 6mm in breadth and 5mm in thickness. Details of the internal anatomy can only be seen after compressing and clearing the fluke. The genital pore is placed to the right of the middle line immediately behind the ventral sucker. The testes are lobed, and situated in the posterior third of the body, and the ovary is on the left side, just behind the ventral sucker. The uterus is coiled and asymmetrical, on the right side. The vitellaria appear as a bands along the edges of the body. The eggs measure about 100cm by 50cm, are ovoidal, operculated and golden brown in color. They contain a fertilized unsegmented ovum when found in sputum or feces.

Life History of Paragonimus Westermani 

When the eggs are in fresh water, at a suitable temperature, a miracidium hatches out in from two to seven weeks. In the Far East the sporocysts, rediae and young carcariae occur in various species of the mollusk Semisulcospira, and especially S. libertine.

The metacercariae are found in various fresh-water crabs (Potamon and Erocheir), and also in crayfish (Astacus). The cercariae penetrate the joint folds and encyst, most commonly, in the gills and muscles.

When the infective cercariae are swallowed by man in uncooked crab or crayfish, they immediately penetrate the wall of the intestine, migrate into the abdominal cavity and pass through the diaphragm into the thoracic cavity where they become adult. An encapsulated cyst develops round the adult worm as a result of inflammatory reaction on the host.

Pathogenicity of Paragonimus Westermani 

The main lesions are in the lung and consist of fibrosis of the pulmonary tissue, cystic dilation of the bronchi and abscesses which ulcerate through to mucous, serous or cutaneous surface. The sputum is viscid and frequently blood-stained; occasionally profuse haemoptysis occurs. The parasite, in its migrations, sometimes settles in abnormal sites such as liver, spleen, brain, the symptoms depending on the location of the fluke.

Diagnosis of Paragonimus Westermani 

The eggs of this parasite are found the in sputum or feces. The diagnosis of pulmonary paragonimiasis can be made presumptively based on compatible clinical manifestations in patients from endemic areas with peripheral eosinophilia and pertinent exposure history. Chest radiographs may demonstrate lobar infiltrates, coin lesions, calcified nodules or pleural effusions.

Adult paragonimids are reddish brown, plump bodied and shaped like coffee beans. They are hermaphrodites and possess large branching testes located in posterior half of the body.

Symptoms of Paragonimus Westermani 

Hardly anyone in the Western world has ever heard of this parasite, but it affects 21 million people in South East Asia. Paragonimus westermani is a fluke, or flatworm that invades human hosts by ingesting raw crustaceans. The parasites migrate from the digestive tract through the lung to cause disease symptoms that often mimic bronchitis and tuberculosis. In some cases the worms encyst in the heart or brain, leading to serious complications.

The disease comes in two forms, either chronic or acute. In the early phase of infection, ingestion of metacercariae produces diarrhea, abdominal pain and fever as the parasites reach and enter the lungs. Eventually, the flukes mature and start making eggs. Some of these are coughed up into sputum, sometimes with blood-tinged sputum, and some may be swallowed and excreted in the feces. Others may be found in pleural fluid (pleurisy).

A number of complications can occur, depending on the organ involved. Pleural effusions can lead to difficulty breathing and chest pain. Cerebral paragonimiasis may be associated with headaches, seizures and other cognitive disorders, and may cause calcification of the skull and brain. The worms may also infect the liver, spleen, skin and abdomen.

The diagnosis is made based on the identification of Paragonimus eggs in sputum or pleural fluid. Ziehl-Neelsen and modified acid-fast staining techniques used for tuberculosis have high sensitivity in diagnosing Paragonimus eggs. Transbronchial biopsy is also helpful. When encested, the worms produce a leukocytic inflammatory reaction that is characterized by eosinophilia. In the brain, they infiltrate the meninges and evoke a granulomatous reaction with hemorrhage and calcification. The worms can also encyst in other ectopic sites, such as the heart and parts of the central nervous system. Infected people living in endemic areas should be tested periodically with a stool test using antigens to detect egg production. The anthelmintic drug praziquantel is the treatment of choice. It is effective in eradicating the worms from the intestinal tract, but it does not kill eggs that are already encysted. Treatment should be continued for up to 20 years.

Stool Examination for Paragonimus Westermani 

The disease is easily diagnosed by identification of the characteristic large operculated fluke eggs in sputum or stool (coughed-up and swallowed). Other diagnostic methods include detection of antibodies to Paragonimus egg antigens using a specific and sensitive antibody test and/or tissue biopsy. The parasite is found in the intestines of carnivorous animals, including cattle and domestic dogs and cats, where it is transmitted to humans through consumption of undercooked crustaceans such as crabs or crayfish. The Centers for Disease Control and Prevention (CDC) recommends praziquantel as the drug of choice to treat paragonimiasis. It can be used in conjunction with pyrimethamine to prevent the development of resistance. Bithionol is an alternative, but is associated with urticaria and other skin problems.

In The Gambia, West Africa, anecdotal evidence suggests that consumption of raw or undercooked crustaceans is common in parts of the country where paragonimiasis is endemic. A recent case report described a 12 year old Gambian boy with pulmonary paragonimiasis who had returned to The Gambia from Casamance, a Senegalese region that is a non-endemic area for the disease. The patient was smear-negative for tuberculosis and neoplasm, but he had high eosinophil counts in sputum. A transbronchial forceps lung biopsy demonstrated a cavitated subpleural lesion, and sputum cytology showed Paragonimus spp. eggs (Figure 4).

Pulmonary paragonimiasis is characterized by productive cough, chest pain, fever and thoracic tenderness. Extrapulmonary symptoms are less common and include acral ecchymoses, gastrointestinal disturbances, abdominal cramping and headache. Cerebral paragonimiasis is also known to occur, and can present with seizures, aphasia, apraxia, and visual disturbances.

The sputum and stool of infected patients are typically bloody, as the parasites are expelled from their cystic lumens. Adult flukes are reddish-brown and ovoid, resembling coffee beans, and measure 7 to 16 mm in length by 4 to 8 mm in width. The worms are hermaphroditic and have two branching testes anterior to a lobed ovary. The ovary is lined with a layer of granulomas, which are visible on smears and in tissue biopsies. A silver stained SDS page gel of soluble P. kellicotti adult worm antigen with a panel of sera from confirmed cases of P. kellicotti infection demonstrates that the antibody response to the antigens at 34 and 21/23 kDa is specific to paragonimiasis (Figure 5).


Symptoms of pulmonary paragonimiasis include chronic cough and chest pain. Patients may cough up coffee-colored or blood-tinged sputum that contains egg clumps. Acute paragonimiasis can cause a pneumothorax (air leak into the lung space) as the larvae migrate through the lungs. Extrapulmonary paragonimiasis can cause disease of the liver, spleen, abdominal cavity, skin and brain. The cerebral form of paragonimiasis presents as obstructive symptoms, such as aphasia, apraxia, blindness and visual disturbances.

Infection with paragonimus westermani is diagnosed by the identification of large operculated eggs in expectorated sputum or stool. Eggs are also found in pleural fluid and the abdomen. A trichuriasis skin test is usually positive but less sensitive than an egg antibody-based assay. The assays use a recombinant antigen from the 28-kilodalton cruzipain-like cysteine protease of paragonimus westermani. The recombinant protein has a high specificity and affinity for the soluble form of the parasite.

Treatment with praziquantel is effective in controlling the pulmonary forms of paragonimiasis. Symptoms typically disappear within a few months and the eggs are cleared from the sputum. However, the resolution of radiographic abnormalities takes months. In addition, praziquantel can be given to prevent paragonimiasis in people at risk for the infection by reducing the time that ingested metacercariae are in the small intestine and the amount of egg-containing feces.

Despite the effectiveness of praziquantel, other drugs are used for treating infections with Paragonimus westermani in areas where the parasite is common. Bithionol is widely used but has side effects, including an increased incidence of diarrhea. Niclosamide is an alternative to bithionol but is less effective.

Infection with paragonimus westermani can be prevented by avoiding the consumption of raw crayfish and crabs in endemic areas. Educational programs should emphasize the need for safer cooking methods and more hygienic food preparation. Prevention programs should also encourage the use of utensils and containers that are free from contamination by fecal matter from the first intermediate host snail. An additional measure that could decrease the prevalence of paragonimiasis is the introduction of a praziquantel-based snail vaccine. Although such a vaccine has not yet been developed, studies are under way to develop a parasiticide that targets the adult fluke.


Paragonimus westermani and other flukes in the genus Paragonimus cause human disease that is similar to tuberculosis. The disease develops in the lungs and sometimes elsewhere in the body. It is usually mild, but can be severe with blood in the lungs (hemorrhage) and other symptoms. Paragonimus is endemic in Asia, particularly southeast Asia, and Africa.

Infection occurs in areas with a high prevalence of human disease, abundant first and second intermediate hosts (snails and crabs or crayfish), and social customs that involve the consumption of raw or undercooked crustaceans. This includes local traditions such as consuming marinated freshwater crab or crayfish to relieve back pain and the consumption of raw crab sauce or jam in China. Infection also may occur in people who consume the meat of carnivorous animal paratenic hosts such as wild boars or deer. Infection has also been caused by consuming contaminated kitchen instruments, including chopping boards, used to prepare raw crustaceans.

The life cycle of the parasite is as follows: When Paragonimus westermani hatches from its egg in a water habitat, it becomes a ciliated miracidium that searches for a snail host. Once a snail is infected, the parasite burrows into its tissues and loses its cilia. It then forms a sporocyst, which releases redia that can asexually reproduce and produce cercariae. The cercariae then invade a crustacean such as a crab or crayfish to encyst within it. When the encysted crustacean is eaten by a person infected with Paragonimus westermani, the cercariae engorge with blood and penetrate the gut wall and diaphragm into the lungs.

Once the cercariae encyst in the lungs, they are deposited in alveoli and then are coughed up or swallowed and expelled with feces. The eggs then pass into the outside environment where they hatch and become metacercariae that invade a crab or crayfish. If the encysted crustacean is then eaten by another person infected with Paragonimus, the metacercariae enter the digestive tract and eventually invade the lungs to encyst in bronchioles. The adult flukes subsequently emerge from the lungs and are coughed up, swallowed and expelled with feces, to enter the outside environment where they hatch again as cercariae and repeat the process.